1. Field of the Invention
This invention relates to the field of antennas; and, more particularly, to external low-profile television HDTV antennas for indoor or outdoor residential and mobile use.
2. Discussion of the Background
Consumer demand for off-air television antennas has been increasing with the interest in direct broadcast satellite service subscription as an alternative to cable television subscription, and the emergence of the new Advanced Television Systems Committee (ATSC) digital television standard adopted by the Federal Communication Commission (FCC) in December 1996. The new standard allows local broadcast television stations to offer either network programming in High Definition Television (HDTV), or multicasting of programming in a digital Standard Definition television (SDTV) format on several side bands. The ATSC standard allows broadcasters to transmit over-the-air digital information at a rate of 19.4 Mbps in a 6 MHz television channel bandwidth in either the VHF or UHF radio frequency (RF) spectrum. Broadcasters have the option of utilizing the majority of the bandwidth for a single HDTV 1080i transmission or for several SDTV transmissions. In addition, over-the-air broadcasters may provide video and data on-demand services providing information and entertainment to subscribers over-the-air as an alternative to receiving information from point-to-point Internet service providers whose data transmissions are limited by network traffic.
Because of the large bandwidth requirement to broadcast 1080i HDTV programming, cable television service providers are experiencing issues in delivering broadcast network HDTV to subscribers in addition to their existing programming. Their “digital cable” services are in reality multiple channels over a community antenna television (CATV) channel bandwidth whose video resolutions are the same as those of analog video signals, significantly less than DVD quality. For this reason, only a handful of cable companies are currently providing a limited number of HDTV broadcast channels to their subscribers while working through bandwidth issues in providing additional HDTV channels. In addition, direct broadcast satellite providers who are able to provide local channels to their subscribers may only do so with the same video resolution as their relative analog broadcasts. In most markets, the only means of receiving HDTV programs on all available broadcast channels in an area is with an appropriate television antenna, and an ATSC-compatible tuner. Because some consumers do not wish to wait for cable companies to work out their bandwidth issues to provide HDTV programming for a monthly fee, a need exists for such consumers to purchase an off-air antenna to receive HDTV programming for free.
In most markets, the majority of ATSC channels available are currently in the UHF television bandwidth (470 to 806 MHz, or television channels 14–69), while continuing their National Television System Committee (NTSC) analog broadcasts on their originally assigned channels. When a high-enough market share owns ATSC-compatible televisions or set-top tuners the broadcasters will then terminate their NTSC broadcast and offer DTV broadcasting exclusively. Broadcasters with NTSC transmissions on VHF low-band (54 to 88 MHz, or channels 2–6) or VHF hi-band (174 to 216 MHz, or channels 7–13) have been given the option to retain their VHF channel for exclusive DTV broadcasting and terminating their UHF transmission, since less power and operating cost would be needed to transmit on VHF to cover the market area than UHF. However, until the time comes, a need exists for an inexpensive UHF television antenna for use by consumers who wish to view broadcast HDTV.
Like analog television tuners, ATSC digital tuners require a proper channel RF signal strength and signal-to-noise ratio (SNR) to ensure a clear, consistent picture. For analog channels, lack of or unnecessarily high signal strength, a high noise floor, or multipath signals reflected off neighboring structures results in snowy, grainy, or ghosted pictures. Most ATSC tuners require a channel signal strength of −18.5 to +15 dBmV with a minimum SNR of 15.2 dB to ensure the tuner receives the data at its maximum rate of 19.4 Mbps with a minimal Bit Error Rate (BER), so that each digital picture broadcast on the 8VSB is displayed with the best possible resolution. Preamplifiers may be used to overcome signal loss due to cable runs and splitters, which is more noticeable on UHF channels than VHF. Conventional 75-ohm input/output preamplifiers have an average noise figure (NF) of 2.9 dB or less. In addition, the noise floor at the receiver is raised depending on impedance mismatch between the signal to the receiver. Such a mismatch is expressed by the Voltage Standing Wave Ratio (VSWR), in which a value of 1 represents a perfect impedance match, and higher positive values indicate a greater mismatch. While an overall bandwidth VSWR of 1 is very desirable, a more realistic VSWR of 1.5 is considered acceptable. Therefore, for good DTV reception, a need exists for a television antenna with a low VSWR to receive a DTV channel with a sufficient SNR. In cases where all the desired digital channels are coming in from the same direction, a need exists for an antenna with an average front-to-back ratio for DTV reception of least 10 dB, since it rejects interfering signals from the sides and back.
Such an antenna would be especially useful in large urban areas where numerous reflecting structures exist; therefore a medium directional antenna is further needed as usually recommended by the CEA for optimal DTV reception in large urban areas.
Ideally, for an antenna to receive the strongest possible signal in a residential area, the antenna should be installed outdoors above the rooftop with as little obstruction toward the TV transmitter as possible. In addition, the antenna should be clear from the power lines that not only could cause electrical shock to an installer or the MATV system, but also man-made noise received by the antenna that would decrease the SNR possibly below the required level, resulting in loss of picture.
Two of the most common types of commercially available outdoor UHF antennas are a log-periodic Yagi and a bayed bowtie array in a vertical plane. Many homeowners are concerned about the physical unattractiveness of such antennas on the roofs of their homes. Such antennas are usually installed indoors. The problem with installing an antenna in the attic is that the signal received by that antenna is at least 45 to 50 percent less strong than the same signal received outdoors. This is due to signal loss through the attic wall or roof material, and if there is masonry, stone, or metal obstructing the signal, that signal is degraded even more or entirely blocked. If that signal loss sends the antenna SNR below the desired level to ensure good reception, the only sure solution is to use a physically larger conventional Yagi or bayed bowtie antenna than what is recommended for outdoor installation, and in some cases the required antenna size may not fit in the attic. Another issue for attic installation is the antenna susceptibility to receive man-made noise from electrical switches, motors, or relays installed in the attic. While man-made noise does not raise the noise floor above the noise figure of the receiver for the UHF channels, it becomes an issue for VHF channels, including low-band, where in some markets DTV is currently broadcast. On such channels, the increase in man-made noise would degrade the SNR for that channel at the antenna, resulting in a potential loss of picture on that channel. If such electrical devices are present in the attic, the likelihood of the antenna picking up the noise increases the antenna size.
Tenants of multi-unit dwellings, including condominium owners, cooperative owners, or renters, install television antennas in areas where they have exclusive use, including a balcony or patio. For this reason, such tenants are able to place Direct Broadcast Satellite (DBS) dishes on their balconies or patios. Rarely are such tenants able to install outdoor television antennas in such areas, simply due to the size of the antenna going outside the boundaries of the areas of exclusive use.
For consumers who want to view HDTV, a need exists for an off-air antenna having good gain, front-to-back ratio, and good VSWR in the operating band, but in an area of optimal reception where the antenna can be safely installed with the fewest obstructions. Such issues become more significant for VHF reception where low-band VHF reflectors on Yagi roof mounts can be as long as 110 inches for optimal performance. In addition, VHF channels are more susceptible to man-made noise effects, so a good signal strength may be necessary on such channels in areas with many obstructions and sources of electrical noise. A need exists for a small, low-profile television reception solution that is easy to install, loosens restrictions on where to install, reject multipath effects in busy urban areas, and has good gain performance to ensure a strong SNR at the antenna.
Research has been done over the years with printed spiral and sinuous antennas for signal reception. DuHamel in U.S. Pat. No. 4,658,262, sets forth a four-element sinuous interleaved circular antenna that showed frequency-independent characteristics and excellent broadband matching. DuHamel derived the design from frequency-independent Archamedies spiral antennas, defined by radial angles, and log-periodic antennas defined by angles, ratios, and adjacent “cells.” The operating bandwidth of the design was dependent on the inner and outer radii of the elements. Such designs have been primarily used for low-profile, millimeter-wave applications in defense and radar. The DuHamel design and other applications of the design used four sinuous elements in a cross-dipole planar arrangement, and feed points for each element to allow dual circular polarization with a 90-degree hybrid feed. The antenna impedance in many applications was about 200 ohms throughout its operating bandwidth, transformed to 50 ohms with a 4:1 impedance transforming balun. In addition, the design allowed a controllable half power beamwidth throughout the frequencies of the operations, with low side and back lobe levels in the radiation patterns.
A need exists to provide a low profile antenna for television reception. To be an affordable television reception solution for consumers, such an antenna would have to be inexpensive to manufacture. While some television stations transmit their analog and digital broadcasts with circular polarization for the purposes of viewers in crowded urban and near suburban areas to receive signals with reduced multipath, acceptable reception of such signals is still possible with a linearly polarized antenna, such as the commonly used high-profile Yagi television antenna.